In the letter, the foreign secretary wrote to the Prime Minister that “it is wrong to see the task as maintaining ‘no border'” in Ireland. Instead, the government should prevent the border becoming “significantly” harder.

Johnson also downplayed the potential effects of the reintroduction of a border, writing: “Even if a hard border is reintroduced, we would expect to see 95% + of goods pass the border [without] checks.” There is an “exaggerated impression” of “how important” the checks at the borders of the EU are, he allegedly wrote.

This is at odds with the government’s official policy of preventing a hard border. Johnson himself has previously emphatically ruled out any kind of hard border in Ireland, saying: “That would be unthinkable, and it would be economic and political madness.”

Labour has called on Theresa May to reconsider Johnson’s position in light of the news. Shadow Northern Ireland secretary Owen Smith said: “Boris Johnson’s comments to the Prime Minister about Northern Ireland are reckless and irresponsible … The Prime Minister should condemn these remarks immediately and seriously consider the position of her Foreign Secretary.”

The leak follows a BBC Radio 4 interview this morning in which Johnson said only that there should not be “excessive checks” at the border.

Johnson was widely mocked after suggested that the Northern Ireland border could be modelled on the congestion charge in London, where those driving into central London are automatically charged for entry.

A government spokesperson told Business Insider that there were “no plans” to model the scheme on London, insisting that Johnson was not “offering a technical solution”.

The latest Brexit leak follows the leaking of new EU proposals for avoiding a border on Tuesday evening.